Field of the Invention
This invention relates to a method for compressing a dynamic range of an image, with which an original image signal representing an original image is processed, and a processed image signal representing an image having a narrower dynamic range than the original image is thereby generated.
Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out, appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material, such as photographic film.
In the image recording and reproducing systems, with which image signals are generated and visible images are reproduced from the image signals, it often occurs that portions of an image, which are to be used and therefore are required to have an appropriate image density in the reproduced image, have image density levels ranging widely from a low density to a high density. Also, it often occurs that what portions of an image having which range of image density are to be used and therefore are required to have an appropriate image density in the reproduced image. In such cases, the image signal representing the original image is processed such that the high-density parts of the original image may have an appropriate level of image density in the reproduced image. Also, the image signal representing the original image is processed such that the low-density parts of the original image may have an appropriate level of image density in the reproduced image. Thereafter, both the images reproduced from the image signals, which have thus been processed in different ways, are displayed side by side on a single display device.
However, if a plurality of images are displayed side by side on a single display device, the problems will occur in that the images inevitably become small in size and therefore hard to observe.
In order that parts of an image covering as wide a range of image density as possible can be used, the level of contrast of the parts of the image having a high or low image density or the level of contrast of the whole image has heretofore been rendered low such that the difference between the highest image density and the lowest image density may become small, i.e. such that the dynamic range of the image may become narrow.
However, if the level of contrast is rendered low, the problems will occur in that details of the image information in the image region, at which the level of contrast has been lowered, becomes hard to observe.
How the problems described above occur will be described hereinbelow.
FIG. 15 is a graph showing an example of how the values of the image signal components of an original image signal Sorg are distributed, which image signal components represent picture elements located along a certain direction (the direction indicated by the arrow x) on an original image. As a whole, the values of the image signal components of the original image signal Sorg are distributed in a step-like pattern along the direction indicated by the arrow x (i.e. the image density of the original image changes step-wise along the direction indicated by the arrow x). Each step part of the distribution of the image density includes a fine change having a comparatively high spatial frequency.
FIG. 16A is a graph showing an example of characteristics with which an original image signal Sorg representing an image is transformed into an image signal Sorg such that the dynamic range of the image may be compressed by lowering the level of contrast of part having a high image density. FIG. 16B is a graph showing how the values of the image signal components of the image signal Sorg' resulting from the transformation with the characteristics illustrated in FIG. 16A are distributed, which image signal components represent picture elements located along the direction indicated by the arrow x on the image.
In this example, the original image signal Sorg having values shown in FIG. 15 is transformed into the image signal Sorg' having values lying on the line A shown in FIG. 16A. As a result, as illustrated in FIG. 16B, the level of image density in the part having a high image density become low. Also, the level of contrast of the fine image structures represented by the fine change in each step part of the distribution of the image density, which step part falls within the high density region, becomes low. Therefore, the fine image structures having a high image density, which image structures are to be used and therefore are required to have good image quality in the reproduced image, become very hard to observe.
FIG. 17A is a graph showing an example of characteristics with which an original image signal Sorg representing an image is transformed into an image signal Sorg' such that the dynamic range of the image may be compressed by lowering the level of contrast of part having a low image density. FIG. 17B is a graph showing how the values of the image signal components of the image signal Sorg' resulting from the transformation with the characteristics illustrated in FIG. 17A are distributed, which image signal components represent picture elements located along the direction indicated by the arrow x on the image.
In this example, the original image signal Sorg having values shown in FIG. 15 is transformed into the image signal Sorg' having values lying on the line B shown in FIG. 17A. In such cases, as illustrated in FIG. 17B, the fine image structures having a low image density become very hard to observe.
Accordingly, a technique for compressing a dynamic range of an X-ray image of the chest of a human body has been proposed in, for example, "Journal of Japanese Society of Radiological Technology", Vol 45, No. 8, p. 1030, August 1989, Mitsuhiro Anan, et al. The proposed technique comprises the steps of:
1) calculating the values of an unsharp mask signal Sus, PA1 2) doubling the values of the unsharp mask signal Sus, the resulting value being clipped at the maximum value (1,023) in cases where the resulting value exceeds the maximum value (1,023), ##EQU1## 3) calculating the values of an image signal representing a reversal image with the formula EQU b=1,023-a PA1 4) adding the products of the values of the image signal representing the reversal image and a coefficient, .alpha., to the values of the image signal representing the original image with the formula EQU c=Sorg+.alpha..multidot.b (.alpha.=0.3) PA1 i) calculating the value of an unsharp mask signal Sus corresponding to each of picture elements in an original image by averaging the values of image signal components of an original image signal Sorg representing the original image, which image signal components represent the picture elements belonging to a predetermined region surrounding each of the picture elements, and PA1 ii) processing the original image signal with the formula EQU Sproc=Sorg+f.sub.1 (Sus) (1) PA1 i) calculating the value of an unsharp mask signal Sus corresponding to each of picture elements in an original image by averaging the values of image signal components of an original image signal Sorg representing the original image, which image signal components represent the picture elements belonging to a predetermined region surrounding each of the picture elements, and PA1 ii) processing the original image signal Sorg with the formula EQU Sproc=Sorg+f.sub.1 (Sus) PA1 wherein the improvement comprises the steps of: PA1 a) calculating the contrast of the original image signal Sorg from the values of the original image signal Sorg, and PA1 b) determining a signal range, to which the function f.sub.1 (Sus) is applied, and/or the level of the value of the function f.sub.1 (Sus) in accordance with the level of the contrast of the original image signal Sorg.
and
The proposed technique has the effects of compressing the dynamic range of an image and keeping the contrast of fine image structures, which are present in each of parts having various levels of image density, high.
However, with the proposed technique, only the dynamic range of parts of the image, which parts have low levels of image density, can be compressed. With the proposed technique, the dynamic range of parts of the image, which parts have high levels of image density, cannot be compressed. Therefore, the proposed technique has the drawback in that it is not suitable for images of bones of limbs, or the like. Also, the proposed technique has the risk that an artificial contour may occur in a visible image reproduced from the processed image signal and adversely affects the image quality of the visible image.
Accordingly, in U.S. patent application 08/225,343 now pending, which is a continuation of U.S. patent application No. 08/093,991, the applicant proposed a method for compressing a dynamic range of an image, comprising the steps of:
where f.sub.1 (Sus) represents a function, the value of which decreases monotonously as the value of the unsharp mask signal Sus increases, whereby the values of a processed image signal Sproc representing an image having a narrower dynamic range than the original image is generated.
With the proposed method, both the dynamic range of parts of the image, which parts have low levels of image density, and the dynamic range of parts of the image, which have high levels of image density, can be compressed. Also, in cases where the differential coefficient of the function f.sub.1 (Sus) is continuous, no artificial contour occurs in the image represented by the processed image signal Sproc. In this manner, the range of image density of the image can be compressed such that parts of the image covering a wide range of image density can be used and may have good image quality in the reproduced visible image, and the image quality of fine image structures at each of parts having various levels of image density may be kept good.
However, with the method disclosed in U.S. patent application No. 08/225,343 now pending, the function f.sub.1 (Sus) for calculating the processed image signal Sproc is fixed for every kind of image signal, and the contrast of the image is not taken into consideration. Therefore, as for an image signal representing an image, which has a wider dynamic range and a higher level of contrast than ordinarily processed images, appropriate compression images, appropriate compression of the dynamic range cannot be carried out, and the contrast of the parts of the image, which parts have high levels of image density, or the contrast of the parts of the image, which parts have low levels of image density, or the contrast of the entire area of the image cannot be lowered appropriately.